Variable reluctance motors are multiphase motors which have tooth-like poles on both the stator and the rotor. There are windings on the stator, but no windings on the rotor. Each pair of diametrically opposite stator windings is connected in series to form one phase of the motor.
Torque is produced by switching current on in the phases in a predetermined sequence so that a magnetic force of attraction results between the rotor and stator poles that are approaching each other. The current is switched off in each pair of windings at a point before the rotor poles nearest the stator poles having that pair of windings rotate past the aligned position; otherwise the magnetic force of attraction will produce a negative or braking torque. The torque developed is independent of current direction. Therefore, unidirectional current pulses synchronized with rotor movement can be generated in a converter using unidirectional current switching elements, such as thyristors or transistors, in each leg of the converter. The current pulses are supplied to the corresponding phase of the motor.
Each time a phase of the motor is energized by closing switches in the converter, current flows in the pair of stator windings of that phase, providing energy from a DC supply to the motor. The energy drawn from the supply is partially converted into mechanical energy by causing the rotor to rotate towards a minimum reluctance configuration. Some of the remainder of the energy produced by the supply is converted into stored energy in the magnetic field, while some is dissipated as core and winding losses. When the switch is opened, part of the stored magnetic energy is converted to mechanical output, and the remainder of the stored magnetic energy is preferably returned to the DC supply by means of a flyback diode pair or other circuitry.
The motor may be run open-loop as in a variable reluctance stepping motor, or may be run closed-loop as in a switched reluctance motor. Furthermore, the motor may be operated such that no two phases are conducting simultaneously (i.e. non-overlapping conduction intervals) or such that some phases do conduct simultaneously (i.e. overlapping conduction intervals).
Current regulators are employed for limiting the phase current amplitudes in a variable reluctance motor. There are several types of current regulators. For example, individual low-resistance current shunts may be coupled to each phase winding to detect the current level in each phase. The output of each current shunt is connected to a separate voltage comparator. Each comparator is also connected to a separate potentiometer for setting the current limit.
Another type of current regulator is disclosed in U.S. Pat. No. 4,595,865 issued to T. M. Jahns on June 17, 1986 and assigned to the instant assignee The cited patent is hereby incorporated by reference. In the current sensing scheme of the above referenced patent, a single voltage comparator performs the current regulation function for an entire power converter. The comparator generates a low logic level whenever any instantaneous phase current exceeds a regulated current limit. Sensor means, which may comprise a plurality of series-connected diode pairs or diode-thermistor pairs, provides voltage signals to the comparator, the voltage signals being proportional to the instantaneous phase current of a respective phase.
Other types of suitable current sensors are well-known in the art, such as: Hall effect current sensors; sensing resistors; sensing transformers; and current sensing transistors, such as those sold under the trademark SENSEFET by Motorola Corporation or those sold under the trademark HEXSense by International Rectifier.
In a typical switched reluctance motor, pole excitation windings on directly opposite stator poles are connected in series aiding to achieve balanced magnetic and mechanical operation. In particular, leakage flux is minimized for maximum utilization of the available ampereturns, and the radial magnetic forces on the rotor cancel, thus resulting in minimum shaft deflection and vibration. Therefore, there are effectively a number of separate circuits equal to one-half the number of stator poles, the circuits being spatially, ohmically, and magnetically isolated from each other. The present invention utilizes the characteristic independence of the motor phase circuits as the basis for a fault-tolerant drive scheme. Such a fault-tolerant motor drive would be particularly useful in aerospace applications, including fuel pumps and electric generators and the like, for which a motor should continue operating, and also be startable, in spite of a phase fault. Another exemplary application is in automotive power steering.